You need internet access to either checkout or update the repo; you can use "go mod download" (or just go build, test, etc.) to fetch the modules too. It's an extra step, but so is vendoring stuff all the time.

But like I said, it's not about "is it useful in some scenarios?" but "is it worth the extra effort?" I'm a big fan of having things be self-contained as possible but for this kind of thing modules "just work" without any effort. Very occasionally you might go "gosh, I wish I had vendored things!", but I think that's an acceptable trade-off.